Due to their strength and high reliability, inductive position sensors are often used in industry and aircraft to determine the position of an object. A typical inductive position sensor comprises a single coil of wire wound on a nonmagnetic bobbin. A target element is moved along an axis that extends through the coil by an object whose position is to be sensed. As the position of the target element changes, the inductance of the coil changes. An appropriate electronic circuit can measure the change in inductance to produce an indication of the position of the object.
Because inductive sensors are often used in environments that are subject to wide temperature variations, they need to be compensated for variations in the resistance of the wire that makes up the coils that occur with changes in temperature. This varying resistance can act to produce inaccurate indications of position unless the coil is compensated. Typically, temperature compensation is provided by having two coaxial coils that are configured to produce a differential signal as the target element is moved. For example, one common inductive sensor design consists of two coils connected end to end along a common axis. As the position of the object changes, the target element moves between the coils, thereby producing a differential output signal by increasing the inductance of one coil and decreasing the inductance of the other. This sensor configuration is compensated for temperature-provided the resistance of the two coils varies in the same way with changes in temperature. The problem with this design is that the length of the sensor is twice as long as the stroke of the target element. This extra length prohibits use of the sensor where space restrictions are critical. Additionally, the length of the sensor reduces its strength, rendering this particular sensor design unusable in areas of high stress.
A variation of the inductive sensor design described above is a sensor having two nested coils. Each coil of the sensor has a conical shaped winding. The two coils produce a differential signal as the target element is moved within the coils. While this design reduces the length of the sensor, the complexity of the coil design makes this sensor design impractical. Also, this sensor can only be operated at relatively low frequencies due to the inherent capacitance of the large number of windings that comprise the overlapping coils.
A third inductive sensor configuration that provides good temperature compensation is the so-called inductive divider sensor. This configuration includes two coils connected in series between a source of AC voltage and a reference potential such as ground. One of the coils has an inductance that varies as a target element is passed through the center of the coil. The output voltage of the sensor taken at a node where the two coils are joined varies with the position of the object. If both coils are constructed of similar materials and are exposed to the same environmental conditions, the resistance of both coils should change equally. Thus the output voltage of the sensor will not be affected by variations in temperature.
While inductive divider sensors provide good compensation for temperature, they have not previously been used to accurately measure the position of an object. This is because it has been impossible to make the sensor produce an output voltage that varies linearly with the position of the object. Therefore such sensors have been used as proximity sensors that determine whether an object is "near" or "far" away from a reference point. However, proximity sensors have not been able to accurately measure how near or far away an object is from a reference point. One example of an induction divider proximity sensor is the commonly assigned U.S. Pat. No. 4,845,429, issued to Burreson.
In light of the problems of prior art inductive sensors, a need exists for a new type of inductive divider position sensor. The sensor should be small and capable of producing a linear output signal as the position of the object changes as well as relatively insensitive to changes in environmental conditions.